The present invention relates to magnetic recording media, for example, for recording and reproducing information in combination with magnetic heads, and particularly to magnetic recording media which produce little noise and exhibit improved corrosion resistance and productivity.
Magnetic disk units are generally used for recording information on magnetic recording media or reproducing and outputting information recorded on the media. In cases of such recording or reproducing, a magnetic head and a magnetic recording medium are generally maintained with a small gap, for example, of 0.2 to 0.3 .mu.m, therebetween. A floating head slider is therefore used for preventing friction and wear caused by the contact between the magnetic head and the magnetic recording medium and/or damage caused by collision between them. Namely, the floating head slider employs the hydrodynamic floating force generated by the speed relative to the surface of the magnetic recording medium in a gap between the head slider and the magnetic recording medium so as to keep the small gap therebetween. On the other hand, in recent years, specifications required for magnetic recording media have been gradually made strict, and magnetic layers which may have high recording density have been required.
Such magnetic layers are usually formed by a method of sputtering a Co-Ni-Pt alloy, which is used as a material for forming the magnetic layers, on a substrate with a Ni-P underlayer therebetween. Although the use of this material enables an increase in coercive force of the magnetic layer formed, it has disadvantages with respect to the generation of great noise and high price. In order to remove such disadvantages, some magnetic layers are formed by forming a Co-Ni-Cr layer on a chromium underlayer on a substrate. In the thusformed magnetic layers, although the cost and noise can be reduced to levels lower than those with the above-described material, it is necessary to form a chromium layer having a thickness of 1500 to 3000 .ANG. as an underlayer on a substrate for the purpose of maintaining a predetermined coercive force of the magnetic layer. There is, thus, a problem in that productivity is deteriorated due to the long time required for forming the chromium layer. Although such a structure exhibits the effect of reducing noise to some extent, as compared with the above-described Co-Ni-Pt layer, noise is not yet reduced to a satisfactory level, and there is a problem with respect to its unsatisfactory corrosion resistance.
Each of such magnetic layers comprises the Co-Ni-Cr, Co-Ni-Pt or Co-Pt magnetic layer which is formed on a chromium or chromium-alloy underlayer by sputtering and has an average crystal grain size of 500 to 600 .ANG..
The magnetic layers are longitudinally magnetized in the direction of a track parallel to the surface of a substrate so that magnetic recording can be made. In order to enable high-density recording by increasing the linear density of a magnetic layer, the coercive force (Hc) of the magnetic layer must be increased. In order to increase the coercive force (Hc), it is necessary to epitaxially grow a cobalt sputtered layer on a chromium underlayer, as disclosed in U.S. Pat. No. 4,652,499. When a cobalt alloy layer is epitaxially grown on a chromium crystal, the grain size of the chromium crystal in an underlayer must be substantially the same as that of the crystal of the cobalt alloy. The chromium crystal grain size is small when the chromium layer is thin, and the crystal grows and the grain size increases with an increase in the thickness. When a magnetic layer is formed by epitaxially growing a cobalt alloy having an average crystal grain size of 500 to 600 .ANG. on a chromium layer, in order to increase the coercive force (Hc) of the magnetic layer formed, the thickness of the chromium layer must be increased so that the crystal grain size of the chromium layer must be the same as that of the cobalt alloy. The thickness of the chromium layer is therefore increased to 1500 to 3000 .ANG. so that the chromium crystal grain size is 500 to 600 .ANG..